1. Field of the Invention
The present invention relates to an active optical coupler for use in an optical fiber data link system and a method for amplifying an optical signal.
2. Description of the Prior Art
FIG. 13 shows an example of a conventional optical star coupler SC applied to an optical fiber data link system having four optical terminals Ta, Tb, Tc, and Td. The optical terminal Ta is provided with a receiving port RXa for receiving an optical signal and a transmitting port TXa for transmitting the optical signal. Other terminals Tb, Tc, and Td are also provided with receiving ports RXb, RXc, and RXd and transmitting ports RTb, RTc, and RTd, respectively. When the terminal Ta transmitted an optical signal PS from the transmitting port TXa toward the star coupler SC, the optical signal PS is distributed into four optical signals PSa, PSb, PSc, and PSd which will be further transmitted therefrom and received by the receiving port RXa, RXb, RXc, and RXd, respectively. Thus, the distributed optical signal PSa which is distributed from the original signal PS returns to the terminal Ta which transmitted the original signal PS.
In FIG. 14, an example of the conventional star coupler SC employed in the optical fiber data link system of FIG. 13 is shown. The star coupler SC includes a mixing rod M having first and second end portions Ma and Mb, a first bundle having "l" number of optical plastic fibers FI which is optically connected with the first end portion Ma, and a second bundle having "m" number of optical plastic fibers FO which is optically connected with the second end portion Mb. When the optical signal PS representing an information is supplied to any of the optical plastic fibers, for example FIl, the original optical signal PS is distributed to "m" number of optical fibers FOl to FOm through which distributed signals PSl to PSm are output, respectively. In this case, the first and second bundles of optical fibers FI and FO function as an input channel and an output channel, respectively.
However, since the optical plastic fiber bundles FI and FO and the mixing rod M have different coefficients of expansion, they expand or shrink at a different thermal expansion rate when the ambient temperature changes, resulting in a big change of the optical characteristics of the star coupler SC. Furthermore, since the mixing rod M is optically connected with the optical plastic bundles FI and FO at the end portions Ma and Mb without any physical connection, the loss of optical signal power at such connected portion becomes large according to the temperature change. Additionally, the vibration coming from the externals displaces the optical fiber bundles FI and FO with respect to the mixing rod M, causing the complete removal of fiber bundles FI and FO therefrom. When the terminals Ta, Tb, Tc, and Td are placed at a widespread area, the optical fiber data link system requires additional optical signal amplifiers on such a long distant optical transmission line therebetween.
In FIG. 15, an example of a conventional active optical coupler proposed in consideration of the above described problems is shown. In this example, the conventional active coupler Ac is used in a 4.times.4 channel communication system which has four input channels and four output channels. The active coupler Ac includes a first photoelectric converter PI having four electric transducers PIa, PIb, PIc, and PId corresponding to input channels, a second photoelectric converter PO having four electric transducers POa, POb, POc, and POd and amplifier AM connected between the first and second photoelectric conversion means PI and PO.
When the optical signal PS coming from the terminal Ta is input to the photoelectric transducer PIa, the optical signal PS is converted into an electric signal ES by the photoelectric transducer PIa and is transmitted to the amplifier AM. The amplifier AM amplifies the electric signal ES and produces an amplified electric signal ES' which will be supplied to the second photoelectric converter PO. By each of photoelectric transducers POa, POb, POc, and POd, the electric signal ES' is converted into optical signals PS'. Thus, the optical signal with a weak power is amplified and distributed to the terminals Ta, Tb, Tc, and Td through the photoelectric transducers POa, POb, POc, and POd, respectively.
However, the active coupler Ac requires "n" number of the photoelectric transducers, causing the increase of cost. It is to be noted that "n" is total number of input channels and output channels of the optical fiber data link system. When the amplifier AM is disabled by such a case that the electric power supply to the amplifier AM is interrupted or the amplifier AM is out of order, the communication, in the optical fiber data link system is also disabled, thereby lowering the system reliability.